Art by firelight? Using experimental and digital techniques to explore Magdalenian engraved plaquette use at Montastruc (France)

Palaeolithic stone plaquettes are a type of mobiliary art featuring engravings and recovered primarily from Magdalenian sites, where they can number from single finds to several thousand examples. Where context is available, they demonstrate complex traces of use, including surface refreshing, heating, and fragmentation. However, for plaquettes with limited or no archaeological context, research tends to gravitate toward their engraved surfaces. This paper focuses on 50 limestone plaquettes excavated by Peccadeau de l’Isle from Montastruc, a Magdalenian rockshelter site in southern France with limited archaeological context; a feature common to many art bearing sites excavated across the 19th and early 20th Centuries. Plaquette use at Montastruc was explored via a programme of microscopy, 3D modelling, colour enhancement using DStretch©, virtual reality (VR) modelling, and experimental archaeology, the latter focusing on limestone heating related to different functional and non-functional uses. While the limited archaeological context available ensures the results remain only indicative, the data generated suggests plaquettes from Montastruc were likely positioned in proximity to hearths during low ambient light conditions. The interaction of engraved stone and roving fire light made engraved forms appear dynamic and alive, suggesting this may have been important in their use. Human neurology is particularly attuned to interpreting shifting light and shadow as movement and identifying visually familiar forms in such varying light conditions through mechanisms such as pareidolic experience. This interpretation encourages a consideration of the possible conceptual connections between art made and experienced in similar circumstances, such as parietal art in dark cave environments. The toolset used to investigate the Montastruc assemblage may have application to other collections of plaquettes, particularly those with limited associated context.

During the experiment the temperature of the fire and the ground at the 35cm and 70cm distances measured from the centre point of the grid were recorded at 10 minute intervals. The fire was fuelled as required to maintain it for 60 minutes, then left to die down. The plaquettes were left in place to cool for 24 hours. These protocols were then repeated, emulating heating and cooling cycles associated with the re-lighting of a fire in the same position. Plaquettes could not be disturbed during the experiment and many were obscured by sediment. As a result, temperature data was not recorded for plaquettes directly and photography was taken only after both heating cycles.

Experiment A: Taphonomy results
Experiment A results are summarised in S.I. Table 1, which describes the physical changes in the limestone in relation to their location and temperature, and S.I. Fig. 3, which shows the observed 4 physical changes in the limestone. The results indicate that the taphonomic scenario is unlikely to entirely explain the pattern of heating and burning on the Montastruc plaquettes. Only plaquettes that had a surface directly exposed to the central temperature of the fire reached high enough temperatures to cross the threshold required to cause observable colour and texture changes in the limestone.  temperature recorded at that position on the grid. Ground temperature is used as a proxy for intensity of heat to which plaquettes were exposed. Physical changes observed in the limestone for each replica plaquette are noted. All temperatures were measured in degrees celsius (°C) S.I. Fig. 3 Photographs showing patterns of heating and burning on the replica plaquettes in direct contact with the fire (SU1, SC1, BU1, BC1) via unmodified photographs (top of each panel) and the same photographs modified via DStretch© (bottom of each panel). Scale bar below each replica plaquette is 8cm in length Where texture and colour changes did occur, this was dramatic and inconsistent with the heating signatures observed on the Montastruc plaquettes. For example, the plaquette (SU1) completely exposed to the fire partially changed to a grey colour with lime (CaO) formation on its upper surface.

S.I.
Lime begins to form on the surface of limestone at around 600°C and the internal structure deteriorates sharply between 700-800°C, causing total pulverisation of the stone [93,94]. This type of discolouration and deterioration of the structure was not evident at Montastruc. Whilst pot lidding and cracking suggested some exposure to high temperatures, both were identified relatively infrequently on the Montastruc plaquettes.
Comparing plaquettes with exposed surfaces to those partially or completely buried demonstrated that a thin layer of sediment insulated the limestone against the heat of the fire. Plaquette SC1, placed on the surface of the ground with the upper surface partially covered by loose sediment (c. 0.5 -1cm in depth) exhibited rubefaction across the upper surface but a grey colour on the exposed edges.
Rubefaction occurs at around 100-300°C, changes to grey between 400 -600°C and superficial lime formation occurs at the 600°C temperature threshold ([89]; see table 2, main text). While the sediment covering was of a minimal depth and uncompacted, this appeared to reduce the temperature to which the plaquette surface was exposed by a minimum of c. 100°C, based on the lack of superficial lime formation despite the temperature of the fire reaching above 700°C. Similarly, the plaquette (BU1) that was buried with the upper surface exposed, exhibited burning and a grey discolouration on the exposed surface, but only rubefaction and sooting on the buried surfaces. Soot is combusted at >400°C [125], further suggesting the buried surfaces were insulated against the extreme temperatures of the fire. The plaquette completely buried (BC1) c. 2-5cm average depth was consistent with these 7 findings, being further insulated from the heat with no evidence of superficial lime formation but exhibiting the presence of soot and rubefaction.
Plaquettes placed c. 35cm away from the centre of the fire did not exhibit any evidence of burning or heating. This is despite the nearest edges of the plaquettes placed at this distance being in proximity to the flames as the fire footprint spread. Plaquettes placed c. 70cm away from the centre of the fire also did not exhibit any evidence of burning, as these were too far away to directly come into contact with the hearth. This suggests that within the archaeological record, plaquettes would have had to be placed in very close contact with the fire to exhibit pronounced evidence of burning. Where heating and burning is evident on these plaquettes, it appears to be pronounced and homogeneous, particularly on exposed surfaces. This particular signature is inconsistent with the heating and burning pattern observed on the Montastruc plaquettes. However, as taphonomy is likely to be highly specific to any given site and collection of plaquettes, it must remain an important factor for any archaeological assemblage.

Experiment B: Boiling stones
To explore whether plaquettes may have functioned as boiling stones, in the manner of similarly identified stones in the Upper Palaeolithic and pre-Columbian North America [103-106], five plaquettes were placed in a fire and heated for 20 minutes; this was sufficient time for the plaquette temperatures to rise to between 150 -400°C and plateau. Observed variations in temperature between plaquettes was linked to their size, with smaller plaquettes reaching higher temperatures than larger examples. On being removed from the fire, the temperature of the edge oriented towards the fire and the centre of the uppermost face of the limestone were recorded. The plaquette was then submerged in c. 5 litres of water contained within a metal bucket for safety (S.I. Fig. 4). The water temperature was measured both before and after each plaquette was submerged to assess the extent of temperature change for the first two cycles. After the plaquettes had sufficiently cooled within the water -defined 8 as when visible steam was no longer produced -they were removed and any visual changes to the limestone noted. The protocol was repeated three times for each plaquette, emulating repeated cycles of use.
S.I. Fig. 4 Photograph showing replica plaquette PB4 after being submerged in water, in this case revealing a dramatic rubefaction

Experiment B: Boiling stones results
The results of Experiment B are presented in S.I. table 2, which summarises the temperature data collected across cycles, and S.I. Fig. 5, which summarises the observed physical changes in the limestone.   Raising water to boiling temperature proved ineffective, but water was heated and maintained to a temperature exceeding 40°C that could itself be useful for a range of activities [107,108]. Heating time was slow, likely due to the volume of water used and the transferring of one stone at a time to allow for the recording of physical changes in the limestone. The temperature of the plaquette had an impact on the increase in water temperature. For example, the hottest plaquette (PB3), with a recorded temperature of 520°C, increased the water temperature by 6°C. Stones of a lower temperature prior to submersion had less impact on the temperature of the water.

S.I.
The use of limestone plaquettes to heat water is technically possible and might have found application within a hunter-gatherer context. However, documented uses are rare except for two specific circumstances -the nixtamalization of maize, and the cooking of starchy foods in hot earth ovens [106,166]. A previous experiment testing whether limestone fragments were possible boiling stones rejected this use after the stone disintegrated and formed a hydrated lime slurry in the experimental cooking water [104]. Alongside this, the shape of the plaquettes would be unusual if selected as boiling stones, as these are typically cobble shaped or rounded, even on the rare occasions where limestone might have been used [103,105].
The action of heating and rapidly cooling plaquettes produced diagnostic heating, burning, and fracturing of the limestone that may be used to identify this activity in the archaeological record. The pattern generated does not appear to match the plaquettes from Montastruc specifically. While limestone may be unsuitable for heating water related to human consumption, this does not preclude heating using this method for other purposes. Heating and rapid cooling of limestone via submersion in water has demonstrable dramatic visual effects. It is possible these properties were recognised and integrated to provide an experiential aspect to the art, further explored in Experiment C.

Experiment C: Water and non-functional activities
In Experiment C, the effect of water on hot plaquettes was explored in terms of its dynamic visual effects, drawing inspiration from observations made during Experiment B and from other Palaeolithic art contexts where the addition of water was used to manipulate material properties; notably the exploding loess figurines reported from the Gravettian site of Dolni Vĕstonice (Czech Republic) [109][110][111]. The heating protocols used in Experiment B were repeated for Experiment C, with a sample of five plaquettes used. Plaquettes were placed in different positions within the hearth, typically with one edge in a closer proximity to the centre of the fire and another positioned towards the exterior to create a temperature gradient across the plaquette. Other plaquettes were positioned in yet closer proximity and orientated with more of their surface exposed to the centre of the heat source.
Once the replica plaquettes reached a temperature plateau during heating, they were removed and water poured onto the engraved surface (S.I. Fig. 6) to explore whether: (1) plaquettes might have been used in this way to harness particular visual effects, and (2) if there were diagnostic differences in heating signatures between cycles of heating and full submersion in water (Experiment B) versus cycles of heating and controlled pouring of water onto the engraved surface only. The cycle of heating and water pouring was repeated three times to emulate extended use and to create comparative data with Experiment B. Any visual effects or physical changes were recorded via photography to allow for comparison with archaeological plaquettes. 13 S.I. Fig. 6 Photograph showing replica plaquette WP3 being doused in water after heating. Note the vibrant colour change that occurs as the water saturates the limestone. Areas of engraving become momentarily more visible as the water rapidly evaporates due to the temperature of the rock.

Experiment C: Water and non-functional activities results
The results of Experiment C are presented in S.I. Table 3, which summarises the temperature data, and S.I. Fig. 7, which summarises the physical changes observed in the limestone.   In keeping with Experiment B, plaquettes reached temperatures ranging from between 150-450°C and colour change was particularly vivid on contact with water. Rubefaction was revealed in a dramatic fashion as the water was poured over the heated engraved surface, washing away debris and producing a 'sizzling' sound as water rapidly evaporated, causing a dramatic plume of steam. The engraved decoration momentarily appeared more vivid as the water evaporated from the plaquette surface.

S.I.
The pattern of burning was varied but with a correspondence to the position of the plaquette within the hearth. In those cases where plaquettes were placed close to the edge of the hearth, the discolouration caused by heating was unevenly distributed across the surface. Edges exposed to the higher heat of the centre of the hearth exhibited a more pronounced colour change. After the first cycle, distinct bands of discolouration were observed on several plaquettes due to these differentials in temperature. Further, the addition of cold water to plaquette surfaces caused rapid cooling, reducing temperatures by up to 200°C. As a result of these rapid thermal fluctuations, particularly after several cycles of heating and cooling, plaquettes exhibited evidence of thermal fractures (see WP3, S.I. Fig.   7). "oven" structure, stacking the limestone to create a semi-enclosed feature within which a fire was lit (S.I. Fig. 8).
S.I. Fig. 8 Photograph showing the semi-enclosed oven structure used for experiment D  (1) to right (6) (S.I. Fig 8). As the fuel obscured the view to some limestone surfaces inside the structure, a sample of visible surfaces across layers was used throughout the experiments.
Refuelling sometimes obscured these data points and a measurement could not be taken. The temperature was recorded at ten minute intervals to explore how effective plaquettes were at retaining heat whilst the fire was active and being regularly fuelled (up to 60 minutes), and after fuelling ceased and the fire died down (70-80 minutes) and went out (at 90 minutes). The structure was left to cool for 24 hours before being deconstructed and visual changes on each plaquette were recorded. The plaquettes used in the experiment created a reference collection that could be compared against archaeological plaquettes to assess the extent of similarity and difference.

Experiment D: Oven structure results
The results of Experiment D are presented in S.I. Table 4, which summarises the temperature data, and S.I. Fig. 9, which details the physical changes observed in the limestone.

S.I. Table 4
Edge facings inside the structure   After the fire died down, the limestone held heat for several hours before returning to near ambient temperature. The flat capstone rapidly increased in temperature and was consistently hot, typically a little over 100°C, which would have been suitable for a range of tasks.
As the plaquettes remained fixed in place, only the internal edges and sides of the plaquettes were exposed to direct heat, resulting in a distinct pattern of heating and burning on the plaquettes.
Discolouration on these internal edges was pronounced, with plaquettes exhibiting rubefaction and pronounced soot build up concentrated around small gaps between the layers of limestone (S.I. Fig.   10). Sooting can provide an indicator of temperature, being burned off as temperature increases (>400°C) [125]. It is likely the surfaces in direct contact with the fire had soot burned away (>400°C) and voids where smoke could escape became a trap for soot deposition. These areas show rubefaction, and when taken with the presence of soot, suggest a temperature of below 400°C. The heat radiated throughout the plaquette, but did not sufficiently heat the outer edges to meet the temperature threshold for a colour change to occur, resulting in a distinct gradation of discolouration across the plaquette. Surfaces not directly exposed to the fire showed no visually recognisable evidence of heating or burning, despite some external surfaces and reaching higher temperatures (<234°C). The result suggests external edges did not meet the c. 250°C temperature threshold for significant discolouration to occur, likely due to heat loss via radiation and the shifting heat source within the structure resulting in fluctuations and localised heating and cooling.
S.I. Fig. 10 Photograph showing partially deconstructed 'oven' hearth structure after the experiment, exhibiting the pronounced soot and discolouration on some plaquette surfaces. The internal edges were sufficiently hot (>400°C) for soot to burn off. Soot was deposited on surfaces below this temperature threshold and closely associated with block morphology, the spaces between blocks acting to channel airflow, in turn encouraging soot deposition. Outer edges furthest from the heat source show no modification The bands of discolouration that resulted from this experiment, especially evident to the bottom layer of stones (D), were comparable to patterns of heating observed for the Montastruc plaquettes, with specific edges and surfaces appearing to be heated to higher temperatures than external edges. The more intensely modified pieces from the top of the structure are less similar to examples from Montastruc. The limestone used in constructing the oven structure required a flat shape for stability.
For Montastruc, a number of the plaquettes might be unsuitable for this purpose, with several being too small or irregular in morphology. The intense sooting observed on some experimental pieces was not observed from examples derived from Montastruc, though it is possible adhering traces of this kind might not be preserved. It is interesting to note that the stacking of plaquettes in making the 22 structure served to entirely obscure the engraved surfaces. This does not preclude engraved plaquettes from such a use; the plaquette recovered from Ètiolles would have been similarly obscured [45].
However, it is unique amongst all experiments in making the art invisible during use in this way, with complex connotations for how the art might be understood, or perhaps suggesting a separation between use of the art and subsequent use in heating. Taken together, it is perhaps unlikely, therefore, that this configuration is responsible for the pattern of heating and burning on the Montastruc plaquettes.

Experiment E: Fireside at night
Parietal and portable art made in limestone caves was often created by lamp light in near darkness, causing particular visual and perceptual effects as the light cast by the lamp flickered across the decorated and morphologically complex cave surfaces [114,115,143,164]. Experiment E was designed to provide a comparison to this experience of art in limestone caves by positioning plaquettes in close proximity to a fire at night, where the light of the fire similarly flickered over the surface of the engraved plaquette. Experiment E carefully positioned a sample of engraved and/or painted plaquettes at the edge of a hearth (S.I. Fig. 11) shortly after dusk, with low ambient lighting conditions. Experiment E was carried out in two phases: a preliminary phase where the experiment was run without collecting temperature data and focusing on visual observation and recording (FS1), and a second phase where the experiment was re-run to focus on collecting temperature data and to test and confirm visual observations (FS2). In both experiments plaquettes were positioned directly around the edge of the fire to form a ring. The fire was lit and fuelled for 60 minutes and observations recorded and documented via photography. The plaquettes were left in place for 24 hours, allowing for the plaquettes to be subjected to a cycle of heating and cooling.

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The results of Experiment E are presented in Table S.I. 5, which shows temperature data from FS2 for plaquettes positioned in proximity to a hearth; S.I. Fig. 11, which shows example visual effects encountered during both rounds of the experiment; S.I. Fig. 12, which details the physical changes observed in the limestone from FS1; and S.I. Fig. 13, which details the physical changes observed in the limestone from FS2. Also pertinent are Supplementary Information files A-F, which show the VR simulations of several of the Montastruc plaquettes modelled around these experimental conditions. S.I.   As the plaquettes remained in place around the hearth, only one or two edges were exposed to the extreme heat of the fire, causing a pattern of burning and heating demarcated by rubefaction in these 29 localised areas of the plaquettes alone. Environmental factors were revealed to be an important consideration in the burning pattern produced on the plaquettes during these experiments. Each experiment was carried out in open air conditions and the wind gave an overall directionality to the heat created by the fire. Smaller scale changes to airflow and shifts in the positions of greatest heat as the experiments progressed was also observed. As reflected in the temperature data in S.I. was the plaquette subjected to the greatest intensity of heat. Rubefaction is evident across much of the surface and soot deposition is evident. These traces suggest the temperature of the plaquette did not exceed c.400°C and it is likely the temperature of the other plaquettes was lower than this, with the greatest intensity of heat localised to the immediate edges contacting the fire. During FS2, FS2.3 showed some of the greatest reactivity to changing light conditions (see Supplementary Information file G), but received the least exposure to heat that would leave a diagnostic archaeologically observable trace. FS2.1 was exposed to the highest heat, in this case above 500°C, burning away soot deposits and leaving a grey discolouration to the edge in proximity to the heat. Across FS1 and FS2, this produced a collection of plaquettes that superficially appear as though several were unmodified by heat, despite all of the plaquettes being distributed in close proximity to each other and to the fire.
The heating and burning pattern across the replica plaquettes from these experiments closely resemble the dominant pattern of heating and burning observed on the Montastruc plaquettes.
Notable visual effects were observed during FS1 and FS2. The flames from the hearth cast light at an oblique angle across the plaquettes, causing elongated shadows that emphasised the natural contours 30 and features of the limestone and increased the visibility of the engraved decoration. However, this effect could be lost where the light was particularly strong after refuelling or where the plaquette was particularly flat and featureless. Equally, where engraving was shallow, the light could wash out and ambiguate a surface, while deeper engraving tended to give a more dramatic effect. This effect brought into focus the relationship between the engraved decoration and the morphology of the support. In some cases, it was clear where the natural features of the support had shaped the form of the engraved motif, for example with legs of an animal depiction being engraved onto natural contours of the support (see FS1.2, S.I. Fig. 11)